Most recreational runners spend their weeks in a moderate middle gear that's too hard to recover from and too easy to drive adaptation. The fix is boring on purpose.
It means roughly 80 percent of your running is done at an easy, conversational effort and about 20 percent at genuinely hard efforts. The middle ground — comfortably hard — is where most recreational runners get stuck.
Easy enough to hold a full conversation without gasping. If you're using heart rate, that's usually around 70 to 75 percent of maximum, though the talk test alone works fine.
Over time, yes. Easy running expands the aerobic base — capillaries, mitochondria, stroke volume — that determines how long you can sustain effort, while keeping fatigue low enough to train consistently.
Usually because they're not actually easy. Fatigue from previous sessions, heat, or simply running above conversational effort all make an easy day feel like work.